Spread-spectrum clocking has been used to reduce Electro-Magnetic Interference (EMI) by “broad-banding” a normally narrow-band clock signal to spread the energy contained therein over a wider range of frequencies. Broad-banding the signal over a wider range of frequencies can reduce the EMI by spreading the frequency component of the clock signal so that, for example, the clock edges of the clock signal become offset from one another in time.
The generation of spread-spectrum clocks can be done using various approaches. Some approaches use a phase-lock loop (PLL) to produce a spread-spectrum output. One such PLL based approach is discussed in, for example, U.S. Pat. No. 6,167,103 to Hardin. In other approaches, a spread-spectrum clock can be provided using delay units. In this type of delay approach, a delay controller circuit can be used to modify a delay provided by an associated cell. A clock signal is provided to a first one of the cells to generate a first output which is then provided in turn to a second cell in the sequence. The output of a final cell can be used to provide the spread-spectrum output. This type of approach is discussed in, for example, U.S. Patent publication No. 2004-0095988.
In still another approach to spread-spectrum clocking, a pseudo random modulation technique is used to provide phase modulation (dithering) of a switching regulator control block to spread the noise-power spectrum in the frequency domain to reduce EMI. In such an approach, and N-bit M-sequence generator can be driven by a clock to produce a pseudo random signal. A shift register is also driven by the clock to delay a pulse width modulation controller output for the spread-spectrum clocking. On such dithering approach is discussed in, for example, Spread-Spectrum Clocking in Switching Regulators to Reduce EMI, by Sadamura et al., Dept. of Electronic Engineering, Gunma University, 1-5-1 Tenjin-cho Kiryu 376-8515 Japan.